Contrary to its name, stainless steel is not literally “stainless steel”. It is called “stainless steel” because it can withstand more time and use before showing signs of wear than ordinary steel.
But is stainless steel really ‘stainless’?
While they are all alloys of iron and carbon (which is what characterises normal steel), which incorporate a good dose of chromium, which is what makes them so corrosion resistant, depending on their atomic structure, and the composition of the alloy (including the addition of more material), they will be more corrosion resistant or will have other properties that make them more suitable for some application or other.
Austenitic stainless steels, for example, are particularly resistant to corrosion, unlike ferritic, martensitic and duplex stainless steels. They are the most common and are achieved by the adequate addition of the austenitic stabilising elements nickel, manganese and nitrogen, which makes them non-hardenable and non-magnetic by heat treatment. The two most common austenitic stainless steel references are the so-called inox 304 and inox 316.
Austenitic stainless steels of the 300 series
Austenitic stainless steels are divided into two main subgroups, the 200 series and the 300 series. 200 series achieve their austenitic structure mainly on the basis of manganese and nitrogen, although they contain a small proportion of nickel, whereas the 300 series is essentially achieved by the addition of nickel.
The 300 series of stainless steels is the largest sub-group of austenitic steels and within this series the most common is Type 304, also known as 18/8 or A2 steel.
Stainless steel 304 and another widely used reference of the same series, stainless steel 316, both contain high levels of chromium and nickel (we will see in the next section the proportion of each to their weight depending on the type of steel), which makes them particularly resistant to corrosion. They also contain the important alloying elements: carbon, manganese, phosphorus, sulphur and silicon, in the same proportion by weight for both types of steel. The rest of the chemical composition is essentially iron.
304 Stainless steel vs 316 Stainless steel: the difference between the two qualities
To the casual observer, the difference between one grade of stainless steel and another is imperceptible, as we have commented before. However, for a fabricator, the difference between stainless steel alloys, such as grade 304 stainless steel and grade 316 stainless steel, can be huge.
The key difference between these two grades of steel is that 316 stainless steel contains molybdenum, in addition to all the alloying components of iron, carbon, chromium and nickel and all the elements described in the previous section. Molybdenum greatly improves corrosion resistance, especially in the most saline or chlorine exposed environments.
This is why 316 stainless steel is often referred to as “acid-proof steel”, a quality that makes it more suitable for applications such as structures near the sea, or even underwater, although 304 stainless steel, with its slightly higher proportion of chromium, has comparable corrosion resistance. In conclusion, although both 304 and 316 stainless steels are suitable for extreme environments, only the latter can withstand prolonged exposure to chlorides.
Applications of 304 stainless steel
As mentioned above, 304 stainless steel is the most common type of austenitic steel. Its nickel content is typically between 8 and 10.5% by weight and its chromium content is typically between 18 and 20% by weight. These high ratios give 304 stainless steel its high resistance to corrosion.
In addition, it contains important alloying elements such as manganese, silicon and carbon, just like stainless steel 316. The rest of the chemical composition is essentially iron.
For outdoor furniture, such as railings and bollards, stainless steel is an ideal material for corrosion resistance, but it will only withstand long-term exposure if the quality is suitable for its environment.
For most environments, 304 is an economical and practical choice, but does not have the chlorine resistance of 316.
Other examples of its application are.
Sinks and backsplashes.
Pots and pans.
Utensils and cutlery.
Architectural panels.
Toilets and sinks.
Plumbing.
Brewing, dairy, food and pharmaceutical production equipment.
Springs, nuts and bolts.
Applications of 316 stainless steel
It is a stainless steel very similar to grade 304, due to its chemical composition which also contains high levels of chromium and nickel.
However, 316 stainless steel contains a significant amount of molybdenum, from 2% to 3%, which makes it even more corrosion resistant than 304 stainless steel and therefore for extreme applications such as marine applications and affects the price of the material.
This slightly higher price of 316 stainless steel compared to 304 stainless steel pays off in areas with high chlorine exposure, particularly in coastal and very saline environments for infrastructure such as coastal roads.
In such applications, 316 stainless steel sheet will last longer than 304 stainless steel sheet, which can mean many additional years of service.
However, its molybdenum content has a detrimental effect on formability.
Common applications for this type of steel include.
-Food preparation equipment, particularly in chlorinated environments.
-Benches and laboratory equipment.
-Coastal building panels, fences and trim.
-Boat hardware.
-Chemical vessels, including for transport.
-Heat exchangers.
-Woven or welded screens for mining, quarrying and water filtration.
-Threaded fasteners.
-Springs.
How do you know if a steel is 304 stainless steel sheet or 316 stainless steel sheet?
You cannot distinguish at a glance between 304 and 316 stainless steel sheets because there is no obvious difference between two identical sheets of metal, one of which is polished or textured in exactly the same way, made of one or the other material. To prove that a stainless steel is 304 stainless steel sheet or 316 stainless steel sheet, a material test report (MTR) is required for the actual material. Some small parts, such as clips and mounting feet, made from 316 stainless steel are specially marked.
304 stainless steel or 316 stainless steel: which one to choose?
Each stainless steel application has its own unique requirements and requires a stainless steel that is designed to get the job done.
In some cases, 304 stainless steel may be the best choice.
Those where excellent malleability is required.
Those where a high degree of malleability is required.
Those with more limited budgets that require an affordable material.
Some situations where 316 stainless steel may be the best choice are.
Environments that include a large number of corrosive elements.
Those where the material will be placed in water or constantly exposed to water.
Those where greater strength and toughness is required.